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Proposed Environmental Factors
A recent CDC funded study on the prevalence of ASD, conducted by the Autism Developmental Disabilities Monitoring (ADDM) Network, states that 1 in 88 children in the United States has an autism spectrum disorder.22 This represents a 78% increase since the release of the first report that focused on ASD prevalence in the United States in 2002.23 There is no question that the diagnostic criteria for autism has broadened and the public awareness for autism has grown, but these changes cannot fully explain the dramatic increase in ASD diagnosis seen worldwide.24 In fact, about 50% of the increase in the diagnosis of ASD remains unexplained.25 Researchers believe environmental factors, which are basically anything influencing the development of the disorder beyond genetic mutations, are playing a significant role in the increase of incidence in autism spectrum disorders.26 However, researchers are still unsure what environmental factors trigger autism, at what point in time these environmental factors affect the child (prenatal, perinatal, postnatal stages), and how these environmental factors interact with our genes.
One of the arguments for the existence of environmental factors that trigger autism is the inconsistency of prevalence rates seen in different areas of the globe. Studies conducted worldwide have shown that on average the prevalence of autism is 1%.27 However, a 2011 study found that the prevalence rate in South Korea was an astounding 2.6%.28 The previously mentioned ADDM study on ASD prevalence in the United States, found that Utah had the highest rate of prevalence at 21.2:1,000, while Alabama had the lowest rate at 4.8:1,000.22 Although these numbers may seem alarming, the difference is likely due to some data collection factors that can affect prevalence, such as the “number of records available, better documentation in records, and increased awareness in communities.”22 Even the results from the South Korea study are attributed to the extensiveness of the study, which attempted to screen every child age 5 to 12 in a population of slightly fewer than 500,000 in a 6-year period.29 These studies show that prevalence rates can be affected by the collection of data, making it difficult to pinpoint how specific environmental factors may be affecting particular populations. However, researchers are beginning to uncover how environmental factors are affecting the development of ASD through examining children’s prenatal, perinatal, and postnatal environments.
Prenatal development is the time it takes for a baby to develop from a single cell after conception into an embryo and later a fetus. During this time exposures to a wide array of environmental factors could influence the susceptibility genes that cause autism. A 2010 study exposed in utero rodents to some of the chemicals associated with an increased risk of autism, such as valproic acid, ethanol, misoprostol, and thalidomide, resulting in irregularities common within autism spectrum disorders.30 Another study tested an antiepileptic drug known as valproate used in utero and found that children exposed to the drug were seven times more likely to develop ASD.31 Another theory explores ultrasounds, an exposure that nearly every fetus endures, as playing a role in the development of autism. Currently, there is no state regulation at all for the use of ultrasounds and recently we have had a “confluence of more ultrasounds, done earlier, and possibly with poorly calibrated equipment and inadequately trained people.”32 A 2010 study found that ultrasounds are “unlikely to increase the risk of ASD.”33 However, there are still research projects dedicated to testing the safety of ultrasounds and their possible link to ASD. Unfortunately, the prenatal period of development isn’t the only time a child can be affected by outside influences to develop ASD.
The perinatal stage of development occurs between before delivery from the 22nd week of gestation through the first 28 days after delivery.34 Many complications associated with delivery can have long term effects that result in the development of ASD. Low birth weight has often been suggested as having a correlation with autism.35 Gestation duration has also been associated with the development of ASD.36 Cerebellar hemorrhagic injury, which is bleeding the brain that injures the cerebellum,1 is also “associated with a high-prevalence of long-term pervasive neurodevelopment disabilities and may play an important and under-recognized role in the cognitive, learning, and behavioral dysfunction known to affect survivors.”37 The International Collaboration for Autism Registry Epidemiology (iCARE), funded by Autism Speaks, is investigating the prenatal and birthing events that may increase or decrease the risk for ASD development.38 A recent meta-analysis of 40 studies identified over 60 possible autism risk factors. Some of these risk factors include fetal distress, birth injury or trauma, low birth weight, multiple births, umbilical-cord complications, summer births, feeding difficulties, and neonatal anemia.39 The analysis also concluded that anesthesia, assisted vaginal delivery, head circumference, high birth weight, and post-term birth were NOT associated with autism.39
Postnatal Environmental Factors
There have been a wide range of postnatal environmental factors proposed as contributors to autism including heavy metals, viral infection, exposure to drugs, chemicals, and vaccines. In fact, the release of a 1999 study, which implicated the MMR vaccine as the cause of autism, created a vaccine scare, the effects of which can still be seen in today’s immunization rates, despite the study's retraction in 2010.40 Thimerosal, a mercury containing preservative that was once present in vaccines and thought to be a contributing factor of autism, was removed from all vaccinations, but the rate of autism continues to rise, indicating that vaccinations are not the cause of autism.41 One of the scary notions pertaining to environmental factors affecting autism is that they potentially number in the thousands and researchers have only scratched the surface of determining which specific factors are causing an increase in ASD incidence.